this is not about any calculations. The suthor is just trying to explain some logics and everybody should apply their minds to understand the rental values in their own city and the cost of buying a new house and then recalculate according to their needs.
Here the author is just trying to give a direction. Its upto individuals to recalaculate based on the city they live.
Even in Mumbai, the rental values and House cost depedns on the location you r trying to live.
